The present invention relates to the field of photodetectors and in particular those used to capture images in different ranges of wavelengths. Photodetectors can thus be disposed according to a matrix configuration to produce an imager.
This invention applies particularly to imaging in the infrared range, and in particular to night vision, earth observation and astronomy.
Present-day photodetectors do not generally provide information about the wavelength of the photons they detect. To obtain such information, one well-known method however involves associating a filter with the photodetector, which restricts the sensitivity of the photodetector to a smaller range of wavelengths. One well-known method involves using in the imagers either a global filter covering all the pixels of the imager and limiting the sensitivity of all the pixels to a same range of wavelengths, or a filter having a color distribution enabling images to be generated in color. Mass-produced color imagers of CCD or CMOS type are thus currently associated with a matrix of filters of Bayer type associating a red, green or blue filter with each pixel of the imager.
The U.S. Pat. No. 5,965,875 describes a photodetector using the intrinsic capacities of semi-conductive materials to let photons penetrate down to a depth depending on the wavelength of the photon, where an electron-hole pair is generated. For this purpose, the photodetector described in this document includes a multi-layer structure, each layer of which has undergone a different doping treatment and is associated with a respective detector circuit. Each detector circuit thus supplies a signal representative of the quantity of photons received by the photodetector in a certain wavelength range.
These photodetectors have the disadvantage of not determining the wavelength of an incident photon, but of working by rejection using filters or by using the intrinsic properties of materials. In an imager, the photons reaching a photodetector, but not belonging to a given range of wavelengths corresponding to the bandwidth of the filter or of the material encountered, are not used to form an image, even though they would be detected by a neighboring photodetector. The light sensitivity of such an imager is thus reduced. This sensitivity is also reduced by the presence of non-transparent structures (oxides, polysilicon, metallizations) covering a considerable portion of the sensitive surface of the imagers. Furthermore, the imagers implementing a matrix of filters with different colors have a spatial resolution equal to the number of pixels of the imager divided by the number of ranges of wavelengths to be discriminated. The result is that the number of ranges of wavelengths discriminated in an imager is generally limited to 2 or 3.
It is thus desirable to produce a photodetector enabling, using a single-pixel structure, the wavelength of an incident photon to be detected. It is also desirable to produce an imager including such a photodetector.